Project Summary Physical inactivity during pregnancy is an urgent public health concern and is implicated in excessive gestational weight gain, gestational diabetes, pre-eclampsia, and preterm birth. Self-reported assessments of physical activity (PA) continue to be the most common method for surveillance, epidemiological, and intervention studies of pregnancy PA given low compliance with activity monitor wear during pregnancy, need for large sample sizes, and the high cost of monitors. Therefore, it is critical that PA questionnaires be both valid and reliable. The Pregnancy Physical Activity Questionnaire (PPAQ) was developed by our group in 2004 (R03HD039341) as the first validated pregnancy PA questionnaire and has since become one of the most widely used instruments for assessing PA in this population. However, the PPAQ was validated using algorithms derived from accelerometers worn on the hip during fixed-duration simulated laboratory activities, which have been shown to perform poorly in free-living settings, particularly in the detection of sedentary and light intensity activity. Since that time, advances have been made in accelerometer and direct observation calibration and validation methods used to determine the validity of self-reported PA and sedentary behavior (SB). This is important as SB has recently emerged as an important new risk factor for adverse pregnancy outcomes independent of PA. Therefore, we are at a key point in time where innovative validation methods can be used to evaluate PPAQ performance in free-living settings and update and refine this widely used self-report tool. Our highly qualified research team will first update the PPAQ to include contemporary SB options (Aim #1). We will then recruit an ethnically diverse prospective cohort of 50 pregnant women in early pregnancy. In each trimester, participants will complete the updated PPAQ, then wear a wrist-worn Actigraph GT3XP-BTLE and a wearable camera over 7-d of free-living activity. Wearable camera images are the best objective method currently available to categorize the context of accelerometer- defined episodes of free-living activity. Using these data, we will: 1) Create a new short-form version of the PPAQ (PPAQ-SF) to reduce participant burden in studies which measure a wide range of other lifestyle factors (Aim #2); 2) Determine the accuracy and precision of the updated PPAQ and PPAQ-SF to estimate time spent in type (e.g., household/caregiving, occupational, sports/exercise, transportation, locomotion) and intensity classification (light, moderate, vigorous) of PA, and SB in a free-living setting (Aim #3). Thus, the updated PPAQ and new PPAQ-SF developed from this work will be the first questionnaires validated in free-living settings to provide accurate and precise estimates of PA and SB in pregnant women. These tools will be critical for: 1) surveillance studies assessing compliance with PA guidelines, 2) etiologic studies of PA and maternal-fetal outcomes designed to identify the optimal dose of pregnancy PA for the prevention of these disorders, and 3) to determine the efficacy of exercise interventions designed to increase PA in pregnancy.